This invention relates to a method and apparatus for ultrasonic imaging, and more particularly to a dynamic electronically controlled aperture in combination with dynamic focusing for improved lateral resolution in sector scanners and other B-scan ultrasonic imaging systems.
One of the most important parameters determining ultrasonic image quality is that of lateral resolution, which refers to a minimum separation at which two targets can be distinguished in the direction of the longitudinal axis of the linear transducer array. Under far field conditions lateral resolution improves as the transducer array aperture increases, whereas under near field conditions lateral resolution improves as the transducer array aperture decreases. It is known that focusing can theoretically establish far field conditions in the near field, so that large aperture would be an advantage at all ranges. However, it is difficult to maintain focus at ranges less than the array aperture even with electronic dynamic focus. In B-scan tomographic cardiac imaging, ranges less than the array aperture are often of interest. Thus, there are important applications where the optimum aperture varies with depth across the image field.
The single-sector scanner is a real time imaging system having a linear transducer array as depicted in FIG. 1, and is described as a cardiac scanner by Thurstone and von Ramm in "A New Ultrasonic Imaging Technique Employing Two-Dimensional Electronic Beam Steering," Acoustical Holography, Vol. 5, 1974, Plenum Press, New York, pp. 249-259. To make a sector scan, the elemental transducers are excited in linear time sequence to generate angulated acoustic beams at many angles relative to the normal to the array at its midpoint. Echoes returning from targets in the direction of the transmitted acoustic beam arrive at the transducer elements at different times necessitating relative delaying of the received echo electrical signals by different amounts so that all the signals from a given point target are summed simultaneously by all elements of the array. In addition to beam steering delays, dynamic electronic focusing to improve image quality is achieved by additional channel-to-channel delay differences to compensate for propagation path time delay differences from a focal point to the various individual array element positions. The beam steering and electronic focusing delays are additive, and the focus can be changed dynamically to increment the range from which echoes are being received during a reception period. In the prior art sector scanners the entire array of receive elements are active during an echo reception period and the received signals from all receive elements are delayed and summed to generate a focused echo signal or video signal. That is, the array aperture during each echo reception period is unchanged and is the maximum possible aperture. At ranges less than the maximum aperture, dynamic focusing delays must be changed so rapidly that it becomes nearly impossible to keep up.